1. Field of the Invention
The present invention relates to an apparatus for measuring an AC electrical parameter of a device-under-test (DUT), such as a resistor, an inductor, or a capacitor, and more particularly, to the apparatus for measuing the AC electrical parameter of the DUT using a measurement signal of desired frequency while applying a DC bias to the DUT.
2. Background of the Invention
A circuit as shown in FIG. 2 (Prior Art) has been used to measure an AC electrical parameter of a DUT. Specifically, a measurement signal (AC) source 206 is connected to one terminal of a DUT 205 with the other terminal, or point G, being connected to one terminal of a current detecting resistor R.sub.r. The resistor R.sub.r forms the negative feedback circuit of an operational amplifier 203 and is coupled to one input of differential amplifier 202 via DC blocking capacitor C1. The other input terminal of this amplifier 203 is grounded; therefore, the other terminal, or point G, of the DUT 205 is virtually grounded. Accordingly, a voltage substantially equal to the voltage of the signal source is applied across the DUT 205. If a voltage V.sub.AD across the DUT 205 is picked up by a differential amplifier 201 and another voltage V.sub.AR across the resistor R.sub.r by another differential amplifier 202, it is possible to obtain the AC impedance of the DUT 205 in accordance with (Rr)(V.sub.AD /V.sub.AR); where V.sub.AD and V.sub.AR are in the form of a vector voltage.
When performing the AC electrical parameter measurement of the DUT 205 by providing a coupling circuit 207 between the signal source 206 and the DUT 205, connecting a DC bias source 208 (which is a voltage source or a current source with one terminal grounded) to this coupling circuit to apply a DC bias to the DUT 205, and using a desired frequency, if the DUT 205 has a high impedance, the value of the resistor R.sub.r is made large to enhance the sensitivity of current detection. But, the output of the DC bias source 208 must be limited within the range where the amplifier 203 cannot be saturated, so that it is impossible to impose a large DC bias on the DUT 205.
To solve the foregoing problem, a circuit as shown in FIG. 1 (Prior Art) has been used.
In this drawing, one terminal G of the DUT 205 is connected to the resistor R.sub.r via a DC blocking capacitor 212 and to an AC blocking inductor 211 which forms the return path of the DC bias source 208. In this circuit, if the frequency of the measurement signal source 206 is low, the capacitor 212 must have a sufficiently large capacitance, especially if the inductor 211 has no sufficiently large impedance in low-frequency measurement, since the result of measurement involves error. This error increase with increasing resistance R.sub.r. On the contrary, if the inductance is small, the equivalent input noise of the amplifier 203 is amplified in a low-frequency range, and further, if the DC resistance of the inductor 211 is large, this causes a DC offset at point G. As a result, an error is involved in the DC bias voltage being imposed disadvantageously on the DUT 205. Accordingly, in the measurement of low-frequency range, the inductor 211 must have a high inductance and a low DC resistance. Hence, the circuit design is difficult to achieve.
A third circuit (not shown) has also been known where current sources I.sub.BS and I.sub.BR of opposite polarity but the same current value are connected to the coupling circuit 207 and the point G, respectively. The bias current flowing into the DUT 205 from, for example, the I.sub.BS is taken out by means of the I.sub.BR so that the DC bias current cannot flow through the resistor R.sub.r. This circuit has drawbacks in that the mechanism of performing the tracking of the two current values becomes complicated; the DC bias current tends to leak to a ground terminal within the DUT and such a leak breaks the balance of current, so that an unbalanced current inevitably flows into the resistor R.sub.r.